In the case of an electronic component provided with an electronic element, an electrode and an electronic element bonding material for bonding them, a solder material is generally used as the electronic element bonding material. The electronic component is produced by bonding the electronic element and the electrode using the solder material. The electronic component is mounted on a substrate such as a motherboard further using another bonding material that is different from the electronic element bonding material. For example, a solder material having a melting point of 200 to 230° C. is generally used as the material for bonding the electronic component, such as a chip inductor, and the motherboard.
When the electronic component is mounted on the motherboard, the electronic component is heated in conjunction with the motherboard chiefly by a reflow device of the hot-air method to melt the solder material having a melting point of 200 to 230° C. In this instance, the temperature of the electronic component reaches 230 to 260° C., and if the electronic element bonding material inside the electronic component is molten, defects may be found in the final product. The electronic element bonding material is therefore required to have a melting temperature that is higher than the maximum temperature the electronic component reaches in the reflow device. Thus, for example, a solder material that is a Pb—Sn alloy containing lead as the main component and about 15 wt % Sn and having a melting temperature of 288° C. has been in use as the electronic element bonding material. However, when a Pb—Sn alloy is used as the solder material, once an electronic device that has as a control means a substrate on which the electronic component is mounted is disposed of and left in a natural environment, there is a concern that lead is released into the soil from the solder material.
With the recent awareness of environmental protection, solder that does not contain lead (lead-free solder) has been in development. For example, a solder material composed of a Pb—Sn alloy having a melting temperature of 200 to 250° C. is being replaced by a solder material composed of an Sn—Ag alloy, Sn—Cu alloy or Sn—Ag—Cu alloy. In particular, a solder material composed of an Sn-3% Ag-0.5% Cu alloy having a melting temperature of 220 to 230° C. is generally used. However, when electronic components are mounted on substrates, the electronic components may be heated to a temperature higher than the melting temperatures of those solder materials. In that case, in the electronic components in which those solder materials are used as electronic element bonding materials, the solder materials may be molten, resulting in a defect in bonding of electronic elements and electrodes.
Moreover, a solder material having a relatively high melting point has been proposed (for example, see Patent Document 1). In Table 1 on page 7 of Patent Document 1 in particular, a lead-free solder material composed of an alloy containing Bi as the main component and a small amount of Ag is proposed. When a small amount of Ag is added to Bi, a eutectic alloy of Bi and Ag (for example, eutectic alloy containing 97.5 wt % Bi and 2.5 wt % Ag (Bi-2.5% Ag)) is generated. The melting temperature of this eutectic alloy is 262° C. Since the upper limit of the heating temperature of the reflow device that performs mounting of an electronic component on a motherboard is about 260° C., an electronic component in which an electronic element and an electrode are bonded using this solder material is considered not to cause any trouble in mounting the electronic component on a motherboard. However, since the temperature of hot air for use in heating is 270 to 300° C., an electronic component having a small heat capacity, such as a chip inductor, needs to have a heatproof temperature about 10° C. higher than the upper limit of the temperature of heating by the reflow device (at least about 270° C.). Therefore, the solder material disclosed in the aforementioned patent publication also cannot be used to bond an electronic element and an electrode in an electronic component that has a small heat capacity.
Furthermore, it is important in the development of lead-free solder materials to reduce the Ag content. It is of requirement that household electric appliances and electronic devices are produced inexpensively. The price of Ag per gram is as high as about 40 yen, and thus the lower the amount thereof used, the more desirable it is. Therefore, also in Sn—Ag alloys, Sn—Ag—Cu alloys and Bi—Ag alloys and the like, efforts have been made to reduce the Ag content to about 0.3 wt % or less or to replace Ag with another element in order to keep the material costs low.
For example, the addition of Zn, Sn, In or a like element to a bismuth alloy that contains Bi as the main component has been investigated. However, the melting temperature of a eutectic alloy composed of 96 wt % Bi and 4 wt % Zn (Bi-4% Zn) is 255° C., the melting temperature of a eutectic alloy composed of 58 wt % Bi and 42 wt % Sn (Bi-42% Sn) is 138° C., the melting temperature of a eutectic alloy composed of 35 wt % Bi and 65 wt % In (Bi-65% In) is 72° C., and a bismuth alloy that has a melting temperature of 270° C. or higher is not yet obtained. Moreover, these eutectic alloys needs attention since these eutectic alloys are generated locally even when the amounts of the additional elements are small.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-353590